The present invention relates to deposition sources and, more particularly, to such sources for providing superior thin films of selected materials.
In recent years, thin film electronic and magnetic devices have become of greater and greater commercial importance. As the need for precision in providing such films has also increased, a number of methods have been developed for more accurately providing such films. These include chemical vapor deposition techniques, molecular beam epitaxy techniques, etc.
The success of such methods depends to a considerable extent on the effusion cell which is the source of the atoms or molecules that are to be deposited on a selected substrate. Such an effusion cell typically has a crucible formed of high purity materials which are able to withstand high temperatures while being maintained in a hard vacuum. A material from which such a beam is to be formed is provided therein usually in a phase other than gaseous. In common uses of such cells requiring the beam-forming material to be converted to a gaseous state, crucible structure materials are chosen that can operate at elevated temperatures on the order of 1500.degree. C. and in a vacuum of typically 10.sup.-10 torr. Both outgassing from such materials, and the decomposition at such temperatures and vacuums, must be avoided to avoid severely contaminating the layers being deposited on the substrate. The presence and density of the various kinds of atoms or molecules impinging on the substrate is directly responsible for the composition of the layer being deposited.
The flux of the atomic or molecular beam impinging on the selected substrate from the effusion cell is a direct function of the vapor pressure of the beam material contained within the cell crucible. This vapor pressure in turn depends on the temperature occurring in that crucible, and depends rather strongly thereon as the flux has an exponential-like relationship to temperature. Thus, fractions of a degree of temperature can make significant changes in the beam flux. As a result, the composition and thickness of layers to be deposited, if they are to be reproducible, require that the crucible be accurately maintained at a constant temperature.
However, there are a number of difficulties in maintaining such a constant temperature in such a crucible and in avoiding contamination from crucible structure outgassing or decomposition. Deposition sources today typically have a serpentine conductive heating element positioned around the crucible at a distance therefrom, and the heating of the crucible will be mostly by radiation in these circumstances with little conduction. Such a heating element is often constrained to have a shape that often does not conform to the crucible shape thereby leading to low heating efficiency. In addition, the heater temperature as a result is going to be substantially higher than that to which the crucible is desired to be raised, a situation which causes added outgassing from the heater element and reduces its lifetime. The non-uniform spatial distribution of the heating elements means that the crucible will have resulting hot and cold zones making achieving of temperature uniformity difficult.
The design alternatives permitted for such a crucible are often limited by considerations necessary in positioning such heating elements around the crucible. The crucible usually has a relatively large opening at the position the beam is to emerge therefrom which results in substantial radiation loss through that opening thereby lowering crucible interior temperatures nearby. This situation leads to uneven heating of the beam-forming material contained within that crucible. Thus, there is desired a deposition source which provides a crucible permitting more uniform temperatures to be maintained therein. In addition, there is a desire to provide a deposition source exhibiting reduced outgassing from its components during use. Further, there is desire for a source which can provide a material beam displaying good directivity. A further concern, in those deposition processes in which a heated substrate is used, is the uniformity of its heating and outgassing from the heater used therefor.